Electro-optical processor having electrical signal writing means

ABSTRACT

An electrostatic non-contacting writing apparatus adaptable for use in a coherent optical processor comprising writing, developing, readout and erasing stations disposed proximate to and consecutively spaced along the direction of motion of a continuous transparent tape. In operation of the processor a time-varying electrical signal is written on the tape in the form of a spatially distributed electrostatic charge pattern by means of a two-step uniform precharging and subsequent partial erasure technique. The tape is then exposed to an oppositely charged cloud of toner particles at the developing station whereupon the electrostatic charge pattern is converted to a visible image of varying transparency by virtue of the toner adhering to selective regions of the tape. Optical processing is performed as the visible image passes the readout station whereat it intersects a laser beam and thereafter the toner image is erased from the tape in readiness for the next cycle of operation commencing at the writing station.

United States Patent McMahon et a].

[451 June 27, 1972 [54] ELECTRO-OPTICAL PROCESSOR HAVING ELECTRICAL SIGNAL WRITING MEANS [72] Inventors: Donald l-lowland McMahon, Carlisle;Allen Raymond Franklin, Concord, both of Mass.

[73] Assignee: Sperry Rand Corporation [22] Filed: Jan. 23, 1970 21 Appl. No.2 5,438

3,045,644 7/1962 Schwertz ..346/74 ES Primary Examiner-Hemard Konick Assistant Examiner-Gary M. Hoffman Attorney-S. C. Yeaton 57 ABSTRACT An electrostatic non-contacting writing apparatus adaptable for use in a coherent optical processor comprising writing, developing, readout and erasing stations disposed proximate to and consecutively spaced along the direction of motion of a continuous transparent tape. In operation of the processor a time-varying electrical signal is written on the tape in the form of a spatially distributed electrostatic charge pattern by means of a two-step uniform precharging and subsequent partial erasure technique. The tape is then exposed to an oppositely charged cloud of toner particles at the developing station whereupon the electrostatic charge pattern is converted to a visible image of varying transparency by virtue of the toner adhering to selective regions of the tape. Optical processing is performed as the visible image passes the readout station whereat it intersects a laser beam and thereafter the toner image is erased from the tape in readiness for the next cycle of operation commencing at the writing station.

2 Clains, 2 Drawing Figures PATENTEDJum ran 3. 873.596

l TONERI{O TAPE I l LIGHT 2 2 INVENTORS MIR OR 001mm H. mun/101v 3 ALLEN R. FRANKLIN v I B) ATTORNEY ELECTROOPTICAL PROCESSOR HAVING ELECTRICAL SIGNAL WRITING MEANS BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to an electrostatic non-contacting writing apparatus adapted for use, for example, in a coherent optical processor.

2. Description of the Prior Art A coherent optical processor typically includes a transparency on which the signal to be processed is recorded in the form of a two-dimensional pattern of discrete segments of varying opacity or thickness for respectively affecting the intensity or phase of a light beam propagated therethrough, the

recording transparency being used in conjunction with additional components such as lenses and spatial filters for performing correlation, pulse compression, spectrum analysis and other functions as is well known to those skilled in the art.

In the early state of the art photographic film was generally used as the recording means because of its many excellent properties, particularly regarding resolution and sensitivity. Photographic film has the disadvantage, however, of not being reusable and further requires considerable development time which precludes its applicability to realtime (instantaneous) signal processing operation. In the interest of overcoming these limitations other recording media have been investigated in recent years. Ultrasonic delay lines and photochromic materials in particular have received considerable attention. Both of these devices are capable of operating in realtime but each has inherent limitations. Ultrasonic delay lines, for example, suffer from high conversion losses, that is, the processed light signal is very weak. In addition, ultrasonic devices are not suitable for processing signals having a time duration larger than the time required for an acoustic wave to propagate from one end of the line to the other, approximately 100 microseconds or less. Photochromic materials, on the other hand, have the disadvantage, compared to other materials, of being extremely insensitive to light, particularly in the erasure mode, to the extent that development of a practical device has thus far been precluded.

Other materials such as ferroelectrics and arrays of electrooptic and liquid crystals have alsov been investigated but research in these areas is in the formative stages and therefore it is not known whether the desired characteristics of reversibility, high sensitivity, minimum delay time, large storage capacity, stability and non-destructive readout will be realized with such materials. These characteristics are attainable, however, in the present state of the art with a coherent optical processor constructed in accordance with the electrostatic technique of the present invention as will become apparent from the subsequent detailed description of the preferred embodiment.

SUMMARY OF THE INVENTION In a preferred embodiment of the present invention a signal which is to be processed is temporarily recorded on a continuous transparent tape in preparation for presentation in the path of a light beam emitted from a laser source constituting a part of the processor. Various means are positioned adjacent the tape in spaced relation therealong for performing a complete operating cycle including precharge, write, develop, process and erase stages which are accomplished successively as a given region of the tape moves past discrete stations whereat the respective means are located. More specifically, at the inception of an operating cycle a precharging electrode establishes a substantially uniform electrostatic charge of predetermined polarity on a given region of the tape which then moves past a writing electrode having the time varying electrical signal to be processed applied to it. As the uniformly charged region of the tape moves past the writing station the charge pattern is spatially modulated in accordance with the time-amplitude variations of the electrical input signal. Thereafter, the electrostatic charge pattern on the tape is converted to a corresponding visual pattern of varying transparency by exposing the tape at a developing station to toner particles having a charge opposite to that on the tape with the result that the toner is attracted to the electrostatically charged segments of the tape in proportion to the charge existing at the discrete segments thereon. The visual signal pattern is then carried across the propagational path of a light beam emitted from the laser source whereupon the optical processing is performed in conjunction with other conventional components referred to hereinbefore, and finally the toner is brushed away from the tape at an erasure station preparatory to the beginning of the next cycle of operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of an optical processor incorporating the electrical writing technique of the present invention.

FIG. 2 is a sectional view through line 2-2 of FIG. 1 for more clearly indicating the relative orientation of the tape and light source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the figures, an electrically non-conducting tape 10 is carried on rotatable aluminum discs 1 1, 12 for rotation in a clockwise direction as indicated by arrow 13 under the control of motor drive means, not shown in the drawings. The tape is preferably of high optical quality, having high transparency and being essentially colorless and sufficiently hard to resist scratching and wear due to abrasion. Clear plastic tape sold under the trademark MYLAR has proved suitable. As the tape moves in the clockwise direction a corona discharge element 14 including a charging wire 16 held at a potential of approximately +5,000 volts and supported in conductive housing 17 operates at the corona discharge level to produce a substantially uniform electrostatic charge of about 1,500 volts on the region of the tape passing thereby. Typically the charging wire extends perpendicular to the plane of the drawing across the width of the nonconductive tape, substantially parallel thereto, and the aluminum disc 11 functions as a grounding member for the corona discharge element. Regarding the aforementioned voltage levels and other voltage levels mentioned hereinafter, it should be understood that they are merely illustrative of a successfully operating embodiment and that other values may be used depending on the characteristics and physical dimensions involved in the spacing and size of various components without departing from the partial erasure writing technique of the invention. I

When the charged tape reaches the vicinity of write electrode 18 positioned adjacent aluminum disc 12 the electrostatic charge is partially erased in accordance with the instantaneous amplitude values of the input electrical signal applied to electrode 18 through step-up transformer 19 and D.C. voltage source 21, disc 12 functioning as a grounding member in this instance. The write electrode preferably has a sharp razor edge and is located a few thousandths of an inch from the tape. In operation, the write electrode reduces the electrostatic charge on the tape to a level approximately equal to the instantaneous value of the input signal. In the case of single channel operation, the electrode edge typically extends across the width of the tape (perpendicular to the plane of the drawing) to form a pattern of parallel lines of varying charge density as the tape moves past. For multiple channel operation a plurality of electrodes can be arranged across the width of the tape, or, if desired, a two-dimensional array of electrodes can be positioned proximate the tape, the array being controlled by standard twodimensional digital addressing techniques. In any case, the tape speed or position can be controlled either continuously or incrementally by means of a magnetic or optical pickoff positioned adjacent the tape in accordance with conventional practice.

The combination of uniform precharging by corona discharge element 14 and subsequent writing by partial erasure has been found to provide significant advantages. In particular, since the write electrode 18 merely removes a static charge from the tape the currents drawn are very small. In other words, the writing procedure is characterized by extremely high input impedance and therefore low power drive circuits are satisfactory for writing. In addition, by using a DC. bias source of approximately 400 volts, linear gray scale recording has been achieved with the appropriate ranges of input signals, 200 to 600 volts being compatible with the previously mentioned voltages. Moreover, it has been found that the resolution is enhanced by this writing technique since redistribution of the electric field lines attendant to a corona discharge is inhibited by the small write electrode current. It should be understood, however, that the optical processor can also be made operational with other writing techniques, for example, writing without precharging the tape by simply applying the signal information directly thereto but only at the expense of degradation of some of the aforementioned characteristics.

After the input signal has been written on the tape it is carried to the developing station 22 where the electrostatic image is exposed to a cloud of negatively charged opaque toner particles (specks of carbon black impregnated in resinous or plastic material of approximately 100 micron diameter or less). The negatively charged toner cloud is provided by means of a canvas belt 23, which has toner applied thereto from hopper 24, moving on rollers 25a, 25b, 25c and 25d into contact with dielectric brushes 26 which sweep the toner from the belt to form the cloud in the region enclosed by baffle 27. Negative charge is applied to the toner by virtue of gentle contact with wire brush prior to being carried into the baffle enclosed region. In passing through the baffle region the nega tive toner particles adhere to the electrostatically charged segments of the tape in proportion to the magnitude of the positive charge on the tape and thereby form a visual image of varying transparency representative of the electrostatic charge image produced by the input signal. At this time then, the input signal has been reproduced in the customary twodimensional format of a photographic transparency and is therefore suitable for use in an optical processor. Hence, in the next stage of the cycle the two-dimensional tape image is carried to processing or readout station 31 where it intercepts the light beam 32 received from laser 33 by way of reflection from mirror 34 as is more clearly shown in FIG. 2. The laser beam is usually collimated and can be increased in diameter to a size commensurate with the area of the pattern on the tape by means of lenses inserted in its path prior to incidence on the tape. Alternatively, the laser beam can be slightly convergent so that upon passing through the tape transparency any undiffracted light 34 will automatically focus to a point and thus be easily separated from the light 35 diffracted by the spatial transparency pattern. Moreover, as will be apparent to those skilled in the art, a pin hole aperture can be inserted at a convenient point in the path of the laser beam to remove local intensity variations of the laser beam thereby assuring more uniform illumination of the tape transparency. Further, it should be understood that a non-laser source could be used as is generally the case in coherent optical processors of the type to which the present invention relates. A laser source is preferred, however, because of its increased brightness and temporal coherence which is required for some envisioned applications. For instance, in a closed circuit television system where the input signal applied to the writing electrode is obtained by scanning a hologram, the tape transparency will be a replica of the hologram and when illuminated with a coherent beam will reproduce an image of the three-dimensional object from which the transmission hologram was constructed.

In the final stage of the cycle, after the optical processing has been completed the tape transparency is moved to erase station 36 where dielectric brushes 37 operating in conjunction with a vacuum (not shown) remove the toner image in preparation for the beginning of the next cycle. Removal of the toner image is facilitated by using a corona discharge to control the charge state of the toner particles relative to the charge remaining on the tape. As indicated in the drawing, this is accomplished by means of a corona discharge element 38 and an additional brush and vacuum setup 39 which are passed in succession after the initial erasure by brushes 37.

It will be appreciated that an aluminum drum having a tape affixed to its circumference could be used in place of the tape connected around the two aluminum discs. ln this case the optical processing step would be performed by reflecting the laser beam from the tape signal pattern. This system, however, requires a highly polished, true cylindrical mirror surface rotating accurately about its center and further requires the use of difi'raction limited cylindrical lenses to compensate for the curved surface of the toner image.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made within the purview of the appended claims without departing from the true scope and spirit of the invention in its broader aspects.

We claim:

1. Apparatus for writing time varying signal information on a non-conductive recording medium in the form of a spatially modulated electrostatic charge pattern comprising,

means for supporting the recording medium in proximate movable relation to precharging electrode means and writing electrode means, said precharging electrode means including a precharging electrode disposed adjacent a side of the recording medium and operative in conjunction with conductive means disposed on the opposite side of the recording medium, said writing electrode means including a writing electrode disposed adjacent a side of the recording medium and also operative in conjunction with conductive means disposed on the opposite side of the recording medium, means for energizing the precharging electrode to establish on a given region of the recording medium moving past the precharging electrode a uniform electrostatic charge distribution of a predetermined polarity at a level capable of producing a discharge across the gap between the writing electrode and the recording medium when the uniformly precharged region thereof moves past the writing electrode held in a de-energized condition, and means for applying to the writing electrode time-amplitude varying signal information of the same polarity and lower amplitude than said uniform electrostatic charge so that when the instantaneous value of the time varying signal is suitably lower than the uniform charge an electrical discharge is produced across the gap between the writing electrode and the adjacent segment of the recording medium causing the charge on said segment to change to a value substantially equal to the potential on the writing electrode thereby spatially modulating said uniform charge by partial erasure thereof to produce on said recording medium as it moves past the writing electrode an electrostatic charge pattern representative of said time varying signal information.

2. The apparatus of claim 1 including electrical biasing means coupled to said writing electrode for providing a bias voltage greater than the peak voltage level of the signal information for linearly varying the spatial electrostatic charge pattern in accordance with the magnitude of the signal information.

* i t i 

1. Apparatus for writing time varying signal information on a non-conductive recording medium in the form of a spatially modulated electrostatic charge pattern comprising, means for supporting the recording medium in proximate movable relation to precharging electrode means and writing electrode means, said precharging electrode means including a precharging electrode disposed adjacent a side of the recording medium and operative in conjunction with conductive means disposed on the opposite side of the recording medium, said writing electRode means including a writing electrode disposed adjacent a side of the recording medium and also operative in conjunction with conductive means disposed on the opposite side of the recording medium, means for energizing the precharging electrode to establish on a given region of the recording medium moving past the precharging electrode a uniform electrostatic charge distribution of a predetermined polarity at a level capable of producing a discharge across the gap between the writing electrode and the recording medium when the uniformly precharged region thereof moves past the writing electrode held in a de-energized condition, and means for applying to the writing electrode time-amplitude varying signal information of the same polarity and lower amplitude than said uniform electrostatic charge so that when the instantaneous value of the time varying signal is suitably lower than the uniform charge an electrical discharge is produced across the gap between the writing electrode and the adjacent segment of the recording medium causing the charge on said segment to change to a value substantially equal to the potential on the writing electrode thereby spatially modulating said uniform charge by partial erasure thereof to produce on said recording medium as it moves past the writing electrode an electrostatic charge pattern representative of said time varying signal information.
 2. The apparatus of claim 1 including electrical biasing means coupled to said writing electrode for providing a bias voltage greater than the peak voltage level of the signal information for linearly varying the spatial electrostatic charge pattern in accordance with the magnitude of the signal information. 